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Precisely Measuring Human Perception – with a Trick

12.03.2018

CITEC team has developed new technique for computer screens

A team at the Cluster of Excellence CITEC has developed a technique that enables to measure visual perception very precisely. Computer monitors have previously posed a problem for this: high temporal resolution screens are very expensive, and conventional monitors cannot be timed finely enough for many measurements. With a trick, though, the team has now succeeded in doing this – by using computer gaming monitors. These monitors are substantially less expensive and with a bit of software reprogramming, they also operate in a very precise way.

A letter appears on the computer screen. At what point does a person actually perceive it? How long does the symbol have to appear to be perceived? This is sometimes not at all clear because it is difficult to measure the stimulus threshold based on the visual presentation. “Many conventional monitors only display images at a certain frequency,” says Dr. Christian Poth, who is a member of the CITEC research group Neurocognitive Psychology.

Letters then always appear for certain time intervals on the screen – for 10 or 20 milliseconds, for instance, on a 100-Hz screen. “In this way, we can only measure very imprecisely how quickly someone perceives something,” says Poth. For such cases, certain mathematical functions can be used to calculate approximated values, but these still require a number of theoretical assumptions that are not directly verifiable.There are high-speed projectors that allow for temporally finer presentation, but these are still extremely expensive. An interdisciplinary team of psychologists and computer scientists at CITEC, however, has succeeded in utilizing conventional gaming monitors in a way that allows visual stimuli to be presented in high temporal resolution.

To do this, the researchers used an LCD monitor that receives temporally flexible images from the graphics card. “We are now able to present images with a variable frequency,” says Poth. The computer screen used has a maximum frequency of 144 Hz, meaning that an image can be displayed at a minimum of 7 milliseconds. “We can now very finely calibrate everything that is presented for longer than 7 milliseconds.”

This makes it possible to display an image for exactly 7.3 or 7.9 milliseconds. “Very new gaming monitors allow for an even more fine-grained process,” says Poth. “There, a presentation time of as little as 4 milliseconds is possible.”The idea for this project was developed by a working group headed by Professor Dr. Mario Botsch (Computer Graphics and Geometry Processing) and Professor Dr. Werner Schneider (Neurocognitive Psychology). “We had already done some joint projects together,” says psychologist Poth. “At a meeting, we discussed this problem we were having with the monitors, and thus the idea was born.”

It took a bout a year until the group had worked out the solution. “Actually, it’s quite easy,” as he explains. According to Poth, it only took a small change in the software: the code reduces the frequency with which the graphics card transmits information to the monitor to a certain value. This causes a new image to appear, for example, at precisely every 7.4 milliseconds instead of every 7 milliseconds. “With this technique, we can now measure perception and the threshold of consciousness in a very precise way.”

But why is it actually so important to know these perception values to such a precise degree? “Our working group focuses on basic research,” says Poth. “For instance, we explore how humans understand the world around them by means of perception in order to act.” According to Poth, precise measurements of visual perception are important in many disciplines. “This includes very basic parameters such as the speed of perception, which is also important in neuropsychology.” It becomes apparent, for instance, when measuring failing brain functions: “This is relevant for individuals who have had a stroke or suffer from dementia.”

In the long term, this new measurement method may hold implications for improvements and adjustments to cognitive interaction technology. “The better one can measure a person’s basic ability to perceive, the better one can adapt technical systems to that person.” says Rebecca Förster, who is part of the research team. In this way, technical devices could be made usable for people with limited visual perception.

A research article about this project has been published in the journal “Behavior Research Methods”. For this project, researchers from the Cluster of Excellence CITEC have been working with Prof. Dr. Ulrich Schwanecke, a computer science professor at the RheinMain University of Applied Sciences.